The present invention relates to fine pitch interconnections and, in particular, to a method for forming fine pitch interconnection materials employing a magnetic mask.
Flip chip semiconductor devices and other leadless electronic components that require fine-pitch interconnections (the xe2x80x9cpitchxe2x80x9d being the center-to-center spacing of adjacent contacts and/or connections and/or features) are becoming more and more important for the more advanced electronic devices because the reduction in physical size that they afford can accommodate the ever-increasing need for more input and output connections and increasing operating frequencies utilized in modern computers and communication equipment. Thus, the ability to deposit interconnection materials such as solder pastes and conductive adhesives at decreasing pitch is becoming more and more desirable.
Stenciling and screening techniques have been employed for depositing solder pastes and conductive polymers have been around and used for more than 60 years, as exemplified U.S. Pat. No. 2,014,524 issued to Franz in 1935. The same basic technique has recently been extended to high precision by employing photo-etching and laser-cutting techniques for forming very small openings.
While some experiments may have employed stenciling and screening techniques to deposit materials with spacing in the range of 50 microns, in actual practice it is difficult to consistently achieve a pitch of about 100 microns. A precise, repeatable, and consistent method is needed to deposit interconnection materials at fine pitch and at low cost, and to do so over a large area as is present on a 6xe2x80x3 or 8xe2x80x3 diameter semiconductor wafer. Each deposit of interconnection material, be it a solder paste or a conductive polymer adhesive, is often referred to as a xe2x80x9cbumpxe2x80x9d as in, for example, a xe2x80x9csolder bumpxe2x80x9d or an xe2x80x9cadhesive bump.xe2x80x9d
U.S. Pat. No. 5,539,153 issued to Schwiebert et al. describes a method of stenciling solder bumps onto a substrate wherein the solder is applied through a stencil/mask and paste method. In the embodiment of FIG. 5, Schwiebert et al employ high temperature AlNiCo permanent magnets placed on the side of the substrate opposite the mask to hold the mask, which is of a ferrous material, against the substrate, thus eliminating the need for frame to hold a stencil. Permanent magnets can be placed on both sides of the mask/substrate sandwich if more attachment force is necessary. Alignment of the mask to the wafer may be accomplished manually using a microscope and tapping the wafer with respect to the mask while aligning the apertures and pads. Alternatively, alignment may be accomplished by using tooling pins and corresponding holes. Automated alignment may be accomplished by using tooling pins or fiducials and a vision system.
The permanent magnets of Schwiebert et al. have dramatic ramifications upon aligning and securing the mask in relation to the substrate and particularly upon separating the mask from the substrate after the deposition of the solder paste material. Alignment of mask to the substrate by the manual method is difficult at best with tapping essentially being a xe2x80x9ctrial and errorxe2x80x9d method. Moreover, this method is inherently rough and is impractical for attaining precision alignment, e.g., in the range of less than about 300 micron. The most severe and intractable problems with the Schwiebert et al. use of permanent magnets are the distortion and/or movement of the mask that occurs during placement of the permanent magnets and, once the mask is held down by the permanent magnets, the smearing of the deposited paste bumps during removal of the permanent magnets and separation of the mask.
In the method of Schwiebert et al. the mask and permanent magnets remain with the substrate until after the solder paste bumps deposited onto the substrate are heated to the melting temperature of the solder paste (e.g., 210-230xc2x0 C., or even as high as 300xc2x0 C.) so as to reflow and attach permanently onto the contact pads before the mask is separated.
The mask of Schwiebert et al. has two purposes: Firstly, to provide a reservoir to control the volume of paste to be deposited, and secondly to act as a dam or otherwise to contain the paste until, and during, the solder reflow process. The solder paste is applied through a stencil/mask and paste method; however, according to Schwiebert et al., the mask cannot be removed without also removing the solder paste (for pitches less than 400 microns) and so must remain in position on the substrate during the solder reflow process. As a result, the mask material must be carefully chosen from the limited number of materials that (1) have a coefficient of thermal expansion that closely matches that of the substrate over a wide range of temperatures, including the high melting temperature of the solder, and (2) are not wettable by the solder paste. In addition, the support structure that holds the mask in position on the substrate must not impose a strain on the mask or the substrate. High temperature permanent magnets are used on one or both sides of the substrate to attach the mask thereto for the purpose of forming a dam for the solder during the solder reflow process.
While Schwiebert et al. mention the use of tooling pins and corresponding holes for alignment, there is no description of the use thereof, and the alignment of the mask and substrate is apparently only approximate in view of the disclosed range of pitches which is limited 150 to 350 microns. Moreover, with the use of permanent magnets, the alignment will easily be disturbed during the placement of the permanent magnet to hold the mask in position after it is aligned with the substrate, apparently then being xe2x80x9ctappedxe2x80x9d into final position. This tapping-in-place process is an inherent problem with the use of permanent magnets and is impractical and/or very costly for the fine spacing and fine pitch, e.g., under 250 microns, of current and future requirements for the deposition of conductive adhesives and even solder paste. In addition, the relative movement of the mask and substrate when they are pressed together under the force of the permanent magnets may introduce minute damage sites onto the substrate that could decrease its reliability or performance.
U.S. Pat. No. 5,046,415 to Oates describes a composite stencil that includes a layer of metallic material such as brass and a layer of flexible material bonded to the metallic layer for the screen printing of solder paste. While the flexible layer of Oates is also used to provide a dam for the solder paste, there is no suggestion that it be held in place with magnets or that it withstand the high temperature of the solder reflow. One of the most difficult aspects of depositing adhesive paste onto a semiconductor wafer is the fact that extremely small amounts of adhesive paste must be deposited onto a multiplicity of very small, e.g., 50-100 microns, contact pads that are situated on a large circular wafer structure, e.g., 6 inches or more in diameter. Alignment of such masks and wafers can generally be achieved only with high precision equipment and a relatively expensive process.
Accordingly, there is a need for an accurate, reliable, repeatable and low cost method of depositing conductive pastes at very fine feature sizes and pitches onto large substrates including the accurate alignment and holding of the mask in position with respect to the substrate. It is also desirable that such method accommodate masks formed of ferromagnetic and similar materials that have a high magnetic attraction to allow fast and accurate deposition of paste adhesives.
To this end, the present invention comprises a method for forming a pattern of adhesive on a substrate comprising:
obtaining a substrate having a pattern of sites thereon on which adhesive is to be deposited;
obtaining a magnetic mask having a pattern of apertures corresponding to the pattern of sites;
positioning the substrate and the magnetic mask proximate each other so that the pattern of sites and the pattern of apertures correspond;
controllably energizing an electromagnet to generate a magnetic field to hold the magnetic mask and the substrate in close contact;
applying adhesive to substantially fill the apertures of the magnetic mask;
controllably de-energizing the electromagnet; and
removing the magnetic mask to leave the adhesive on the pattern of sites.
A magnetic mask according to the present invention comprises a thin metal membrane and a layer of compressible material having a surface energy less than about 50 dyne-cm formed on at least one side of the thin metal membrane. One of the thin metal membrane and the compressible material has ferromagnetic properties, and the magnetic mask has a pattern of apertures.